The present invention relates to sol-gel methods for the manufacture of ceramics, and more specifically to methods useful for the sol-gel synthesis of ceramics comprising alkaline earth metal oxides, particularly ceramic films or coatings comprising these oxides.
While the production of oxide and other ceramic products through the sintering or reaction sintering of appropriate inorganic starting materials has long been practiced, recent advances in materials science have demanded that new methods for making ceramic components be developed. Particularly in the area of glass or ceramic films or coatings, techniques such as vapor deposition and sol-gel processing have been the subject of increased attention. These processes have the potential for producing high quality ceramic materials at reduced temperatures, and thus can be used, for example, to impart physical protection, increased surface hardness, special dielectric properties, or other advantageous properties to a variety of substrate materials.
One specific example of such a development is found in U.S. Pat. No. 4,935,387, which discloses methods for the production of mica coatings on inorganic fibers. As disclosed in that patent, synthetic fluormica coatings may be developed on fiber or other substrate materials from acidified alkoxide sols, applied to the substrates as coatings and then gelled and heat treated to to develop the desired fluormica crystal phases.
The sol-gel method described in this patent is particularly suitable for the production of potassium-containing fluorphlogopite micas. However, micas with alkaline earths, rather than potassium, as the interlayer cations are desirable for their stronger interlaminar shear properties and for their greater refractoriness. The method of the aforementioned patent is not well adapted to the synthesis of such micas.
In general, homogeneous sols and gels of the alkaline earth metals are not easily prepared from acidified alkoxide solutions of alkaline earth metals when fluoride is also present, because the addition of fluoride usually results in the precipitation of alkaline earth fluorides. Thus although some calcium phlogopite mica products have been obtained by this method, better approaches to the production of strontium and barium micas are needed. In the case of barium alkoxide solutions, particularly, barium precipitates can be formed even in the absence of fluorine from the solution. These difficulties also extend to the synthesis of other ceramic materials by sol-gel processing.
Accordingly, it is a principal object of the present invention to provide a method for the production of stable, homogeneous alkaline earth-containing sols useful for the synthesis of ceramic products from solutions.
It is a further object of the invention to provide a sol-gel method for making a ceramic product, including a ceramic film or coating, from an alkaline earth-containing sol.
It is a further object of the invention to provide a novel sol-gel method for the synthesis of alkaline earth metal-containing fluormicas which employs new solution complexes of alkaline earth metal sol components for improved sol stability.
It is a further object of the invention to provide fiber materials provided with alkaline-earth-containing ceramic coatings, and products incorporating such coated fiber materials.
Other objects and advantages of the invention will become apparent from the following description thereof.